The role of mercury in the dissolution of aluminium sacrificial anodes: A study using ion implantation

Implantation of Hg⁺ ions into Al was used to produce, on Al, surface alloy layers consisting of a metastable AlHg solid solution. The anodic behaviour of these surface alloys was examined using potentiodynamic polarization, current-time transients at constant potential and galvanic coupling with steel. The electrochemical studies were supplemented by scanning electron microscopy and glancing-angle Rutherford backscattering measurements. The results obtained indicated that the presence of Hg in Al, even when homogeneously distributed in metastable solid solution, produced an alloy which was extremely reactive, electrochemically, and which had little immediate tendency to passivate. The reactivity was not associated with the grain boundary regions; the whole alloy dissolved more or less uniformly. The effects of implanted Hg on the dissolution of Al were relatively persistent, although they were eventually lost. The implications of these results are considered in the context of the role of mercury in the dissolution of commercial AlZnHg sacrificial anodes. In the latter materials the mercury is thought to be located primarily as a second phase in the grain boundaries but, although they exhibit some preferential grain boundary corrosion, their grain interiors also dissolve readily.